This invention relates to a superconducting magnet assembly for a magnetic resonance imaging system (hereinafter called "MRI"), and more particularly to compensation for magnetic interference from mobile vans or vehicles in which the system is transported.
As is well known, a superconducting magnet can be made superconducting by placing it in an extremely cold environment, such as by enclosing it in a cryostat or pressure vessel containing liquid helium or other cryogen. The extreme cold ensures that the magnet coils are superconducting, such that when a power source is initially connected to the coil for a short period, current will continue to flow through the coils even after power is removed due to the absence of electrical resistance, thereby maintaining a strong magnetic field. Superconducting magnets find wide application in the field of MRI.
In a typical MRI magnet, the main superconducting magnet coils are enclosed in a cylindrical pressure vessel, contained within an evacuated vessel and forming an imaging bore along the axis. The main magnet coils develop a strong magnetic field in the imaging bore.
However, it is necessary for quality imaging to provide and maintain a highly homogeneous magnetic field in the imaging region. This has proven to provide problems for superconducting magnets which are suitable for, and used in, a variety of types of mobile vans in addition to more conventional fixed diagnostic clinics or hospital installations.
The magnetic field homogeneity in a mobile superconducting magnet is affected by the magnetic materials, such as carbon steel or other magnetic materials, around or in the vicinity of the magnets. Since a mobile van can have carbon steel in the floor, walls and roof, the magnetic field homogeneity of a MRI magnet will be distorted when it is installed in a mobile van. This magnetic field distortion is usually applied to a few inhomogeneity harmonics which are the decomposed components of the magnetic field. This provides difficulties in the magnet shimming processes normally utilized for obtaining a highly homogeneous magnet field, and often for mobile vans requires excessive amounts of magnetic passive shimming materials to be installed in the warm bore of the magnet. This may even result in a magnet that cannot be shimmed by conventional means.
An excessive amount of magnetic shims in the warm bore can cause several problems. Space in the warm bore is very limited preventing the installation of the large number of shims which may be required to provide the required imaging magnetic field homogeneity. Also. the installation of large amounts of shim materials requires a long shimming process, increasing the labor cost. In addition, the material properties of shims can be affected by temperature change and the temperature change in the warm bore is very large due to the activation of the electronics circuits and coils in the bore such that, the large amount of shims required in the bore would result in large magnetic shim property change due to the temperature change.
Still further, it is common for a particular superconducting magnet design or model to be carried by more than one type of mobile vehicle in addition to using the same design in fixed hospital or diagnostic clinic installations. It is highly desirable to be able to readily and effectively utilize the same production design of superconducting magnet in those various but different environments in which the magnetic material around the superconducting magnet varies.